1. Field of the Invention
The present invention relates to a thermal printing head, and more particularly pertains to a thermal printing head of thin film type which employs a resistive element for heat generation.
2. Description of the Prior Art
Recently, a thermal printing technique has been applied, as a recording method which is maintenance free, to various recording devices such as a recorder, a printer and facsimile. Thermal printing is accomplished by bringing a thermally sensitive recording material, such as a paper provided with a coating that will turn dark when heated, into intimate contact with the thermal printing head and impressing a voltage by means of electrical supply conductors across the electrically resistive elements to form the desired symbol. For this thermal printing, various kinds of thermal heads have been developed. They can be broadly classified into three types, namely a thin film type, a thick film type and a silicon planar type. Among these three types, the thermal printing heads of the thin film type have bright prospects in view of the fact that they can be used in not only the devices of a popular type but also of a high-grade type because the resistive elements thereof may be made small.
In FIG. 1, there is shown the structure of a heat-generating element in a conventional thermal printing head of the thin film type. Referring to FIG. 1, the thermal printing head has a support member 1a of aluminum oxide or other heat-conductive material which serves as a heat sink and a flat substrate 1b, of highresistivity material, such as glass, which is attached to the support member 1a. Upon the substrate 1b are deposited, successively, a resistive layer 2 which serves as a heat-generating means, a low electrical resistive layer 3, such as double layer of chromium and gold, for the interconnection conductors or leads, protective layer 4 to protect the resistive layer 2 from oxidation, and an abrasion resistive-layer 5 to protect the resistive layer 2 from abrasion when the thermal head is brought into contact with the thermally sensitive paper. In the structure as shown in FIG. 1, tantalum nitride is often used for a material of the resistive the layer 2. Tantalum nitride is well known to show very stable characteristics when used as the resistive material of integrated circuits of the thin film type.
However, even if such a material is used, it is necessary to provide the protective layer 4 for protection of the resistive material from the oxidation which occurs because the thermal printing head is used at a high temperature. The reason why the protective layer is required is that tantalum nitride film have to be formed thin, on the order of about 500 A, to obtain an electrical resistance of about 100.OMEGA..
Generally, a heat-resistive material for use in the resistive layer has an oxidation-resistive characteristic owing to the fact that the oxidation layer formed over the surface of the material at the time of its initial use prevents the other portion from the further oxidation. In the thermal printing head, because the thickness of the tantalum nitride film is not sufficient to allow for the formation of an oxidized layer, it becomes necessary to cover the resistive material with a protection layer.
As for a resistive material other than tantalum nitride, there are known various kinds of interstitial alloys such as nitrides, carbides, boronides and silicides, and various kinds of materials used for heaters such as nichrome alloys. However, these materials have almost the same specific-resistivity as that of tantalum nitride, so that it is also necessary to form a protective layer thereon.
In order to solve the problem as described above, there may be adopted a method in which a relatively thick layer is formed and the surface portion thereof is oxidized by a heat treatment so as to prevent the oxidation. This method is known as the method of adjusting the value of electrical resistance of a TaN.sub.2 layer used in an integrated circuit of the thin film type. However, this method is not available for practical use because it is difficult to obtain a large number of the elements with the same electrical resistivity, and the process for making the thermal printing head is troublesome.